1. Field of the Invention
This invention relates to a switching-system constant-current power supply for supplying a stable current to an intermittent load.
2. Description of the Related Art
When using a light-emitting diode (hereinafter “LED”) as a light source, the current supply to the LED must be stabilized in order to obtain a constant amount of emitted light, brightness, and the like. As an example, FIG. 4 shows the constitution of a switching constant-current power supply, conventionally used for supplying this type of stabilized current to a load.
In FIG. 4, reference numeral 1 represents an input terminal for receiving power supplied from an external battery, and reference numerals 2a and 2b represent output terminals for stably supplying predetermined current to a load 6 having a plurality of LEDs, connected between them. A power converter 3 is connected between the input terminal 1 and the output terminal 2a, and forms a voltage-boosting chopper converter comprising a choke coil L1, a switching transistor Q1, a rectifying diode D1, and a smoothing capacitor C1.
A detector 5 is connected between the other output terminal 2b and ground, which serves as a reference potential point of the circuit, and detects the current flowing to the load 6 (hereinafter “load current”) and generates a feedback signal in correspondence with the load current. A controller 4 is connected between the power converter 3 and the detector 5, and drives the power converter 3 in correspondence with the size of the feedback signal, received from the detector 5.
The power converter 3, the controller 4, and the detector 5 form a switching constant-current power supply.
Explained simply, in the operation of the switching constant-current power supply of FIG. 4, the controller 4 turns the switching transistor Q1 on and off by using an on-duty corresponding to the feedback signal from the detector 5. The size (signal amount) of the feedback signal corresponds not to the output voltage, as in a conventional switching constant-current power supply, but to the output current (=the load current). For this reason, the switching transistor Q1 performs its on-off operation at an on-duty corresponding to the load current. For example, when the load current is lower than a stabilization target value, the voltage between the terminals of the smoothing capacitor C1 is boosted to increase the load current. As a result, the load current of the device in FIG. 4 is stabilized.
Some recent display devices and lighting devices using an LED as a light source aim to conserve power and the like by repeatedly switching the LED on and off at high speed. In such devices, there is inevitably a period when current is flowing to the load (hereinafter “current flow period”) and a period when current is not flowing to the load (hereinafter “current cut-off period”). In a switching constant-current power supply where the power for supplying current to the LED has the constitution shown in FIG. 4, the feedback signal from the detector 5 to the controller 4 is approximately zero during the current cut-off periods. In other words, the feedback signal is a pulse rather than a direct current.
Normally, the constitution of the controller 4 is not one which can handle a pulse-shaped feedback signal. When this type of pulse-shaped feedback signal is supplied to the controller 4, during the current cut-off period, the controller 4 attempts to maximize the on-duty of the on/off operation of the switching transistor Q1, and, during the subsequent current flow period, the controller 4 attempts to minimize the on-duty of the on/off operation of the switching transistor Q1. When the on-duty reaches its maximum during the current cut-off period, the voltage between the terminals of the smoothing capacitor C1 abruptly increases, and, in the subsequent current flow period, there is a possibility that a load current greater than the stabilization target value will flow for a comparatively long time.
When the feedback signal is pulse-shaped, it can, for example, be smoothed by a capacitor or the like, and supplied to the controller 4 as a direct current. However, when the pulse-shaped feedback signal is smoothed by a capacitor or the like, the size of the feedback signal at the moment it flows to the controller 4 is approximately equal to an average value over a predetermined period of time. Consequently, in the event of an interrupted non-cyclical load or other such load fluctuation, a long time is required before the load current, which has deviated from the stabilization target value, returns to the target value.
With an intermittent load, the response speed of the control operation along a feedback loop via the choke coil L1, the smoothing capacitor C1, the load 6, the detector 5, and back to the controller 4, is insufficient to keep up with changes in the load, resulting in the possibility of unstable load current.